On January 1, 2020 will happen ABSOLUTELY NOTHING.
It will be gradual, as long as you're developing more and more packages will refuse to work. If you realize there's a bug in one of your dependency and the fix is in a version that doesn't work on python 2, tough luck you either will have to backport the fix or migrate the code.
As time progress it will be more and more work to deal.
And guess what, some packages didn't even want to wait they already dropped python 2 support: https://python3statement.org/ (look at Projects Timeline) so it is starting happening right now. I'm wondering if pip will decide to drop the support in 2020, that might end up being the biggest hit.
On January 1, 2020 will happen ABSOLUTELY NOTHING.
It will be gradual, as long as you're developing more and more packages will refuse to work. If you realize there's a bug in one of your dependency and the fix is in a version that doesn't work on python 2, tough luck you either will have to backport the fix or migrate the code.
As time progress it will be more and more work to deal.
And guess what, some packages didn't even want to wait they already dropped python 2 support: https://python3statement.org/ (look at Projects Timeline) so it is starting happening right now. I'm wondering if pip will decide to drop the support in 2020, that might end up being the biggest hit.